Plasma cells are the terminally differentiated cells of the B lymphocyte lineage whose sole purpose is to produce antibody molecules. Throughout the differentiation process, B lineage cells display cell surface molecules which, in addition to providing specific functions to the B cell, also serve as landmarks for the developmental stage of a B cell or can divide B cells into functional subsets. In an attempt to generate monoclonal antibodies to different subsets of B cells, C57Bl/6 mice were hyperimmunized with DBS/2 spleen cells and hybridomas were produced. One monoclonal antibody appears to recognize an activation-induced antigen on both B cells. This molecule is expressed on only a small amount of unactivated B cells in the spleen but is found on 50- 100% of LPS-stimulated B cells, 24% of anti-IgM plus IL-4 stimulated B cells and 60% of IL-5 activated cells. This antigen is expressed at higher levels on unactivated thymocytes than peripheral T cells and, with ConA stimulation, it is found on 45% of thymocytes and only 4-16% of peripheral T cells. Immunprecipitation reveals a protein in the 12-18 KD range on both B and T cells. To date, the characterization of this antigen suggests that it has not been previously described. Production and screening of an expression library is underway. The mechanism that produces antibodies is largely understood in that different gene segments are combined to create an antibody with specificity for a particular antigen. The details of the rearrangement process which combines the gene segments and the process of and the process of "choosing" which gene segments to combine, however, are still not understood. We are studying in detail, a small light chain gene family, Vk10, which is known to be utilized in a number of different antigenic responses. Sequence analysis reveals no structural reason preventing expression of this gene. PT-PCR experiments indicate that there is expression of this Vk gene in the spleen. The levels of this particular Vk10 gene are being quantitated in comparison to the other members of this family. Expression at different developmental time points will also be quantitated.